Sensor device for electrocardiogram

ABSTRACT

A sensor device intended to be implemented at the vicinity or inside a nuclear magnetic resonance apparatus, and more particularly on a patient inside the canal of the magnet of a magnetic resonance imager (MRI). The device includes at least two non-metal electrodes intended to be applied to the skin of a patient, and an electro-optical conversion, amplification and filtering module for the electric signals received from the heart by the electrodes. The module is arranged in a shielded casing forming a Faraday cage, and optically connected to a display and/or monitoring apparatus. A support body made of non-magnetic material carries the electrode and the shielded casing containing the module.

BACKGROUND OF THE INVENTION

The present invention relates to the field of receiving and measuringbiological signals and the surveillance of patients, particularlypatients undergoing nuclear magnetic resonance (NMR) examination forexample in a magnetic resonance imager (MRI), and has for its object adetector device for electrocardiograms.

BACKGROUND OF THE INVENTION

At present, electrocardiogram signals from patients subjected to an NMRexamination are received by means of electrodes with metallic conductorsand transmitted to a visualization and/or surveillance apparatusdisposed in the shielded enclosure or outside, by means of electriccables of great length, as the case may be shielded and twistedtogether.

However, these electrical cables, forming antennas, disturb theelectromagnetic environment of the NMR apparatus and render false themeasurements and, in the case of an imager, the virtual reconstructions(images) obtained by this latter.

Conversely, the field gradients, the radiofrequency fields and thephenomena connected to switching between emitting and receiving coils inthe course of an experiment of the NMR type greatly disturb thetransmission of the low power signals received at the heart and may, bygeneration of important false images, render these latter totallyunusable, even though the patient is disposed within the interior of theprincipal magnet of the NMR apparatus.

Moreover, possible movements of the patient (particularly breathing)give rise to movements of said electrical transmission cables in theexisting field, from which automatically results an induction ofvoltages generating false images.

Moreover, the troublesome phenomena described are greatly amplified whenthe transmission cables have one or several loops.

Thus, the energy stored by the cable or cables, subjected to highintensity electromagnetic fields, at said loops or cable, can be fairlygreat so as to provoke substantial heating of said cable, which can giverise to burns on the patient's skin on which a portion of the cablesrest, particularly when passing through an emission antenna.

Moreover, the placement of the electrodes on the patient in the regionof the heart requires, for correct emplacement, the intervention of aspecialist, each of said electrodes having to be precisely positionedindividually.

It has been proposed, in an attempt to overcome certain of thesedrawbacks, to use algorithms for correcting the false images generatedby the gradients and the radiofrequency fields.

However, these algorithms are not adapted to other than a predeterminedtype of NMR apparatus, with a given configuration of windings andfrequently with a particular NMR sequence, which results in a great lossof flexibility during their application.

Moreover, they do not permit solving the problems of burns, nor theproblems of induction of voltage because of the movements of the cablesor again the problems of positioning the electrodes.

SUMMARY OF THE INVENTION

The present invention has particularly for its object to relieve all ofthe recited drawbacks, by simple means, not burdensome and of generalapplication, no matter what the type of apparatus used.

To this end, it has for its object a detector device forelectrocardiograms, adapted to be used in a charged and sensitiveelectromagnetic environment, particularly adjacent or within an NMRapparatus, and more particularly on a patient within the tunnel of themagnet of an NMI, characterized in that it is principally constituted,on the one hand, by at least two non-metallic electrodes, adapted to beapplied to the skin of a patient, on the other hand, by a module forfiltering and amplifying and electro-optically converting electricalsignals from the heart, received by means of said receiving electrodes,disposed in a shielded casing forming a Faraday cage and connected byoptical means to a visualization and/or control apparatus and, finally,by a support body of an amagnetic material, bearing the electrodes andthe shielded casing containing the module.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description,which relates to a preferred embodiment, given by way of non-limitingexample, and explained with reference to the accompanying schematicdrawings, in which:

FIG. 1 is a schematic view in side elevation of the detector deviceaccording to the invention;

FIG. 2 is a functional schematic diagram of the detector device shown inFIG. 1;

FIG. 3 is a schematic representation showing the use of the detectordevice shown in FIG. 1;

FIG. 4 represents an electrocardiogram signal obtained by means of adetector device according to the invention, and

FIG. 5 represents an electrocardiogram signal obtained by means of aconventional detector, under conditions identical to those that producedthe electrocardiogram of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention and as shown in FIGS. 1 and 2 of the attacheddrawings, the detector device is principally constituted, on the onehand, by at least two non-metallic electrodes 1 adapted to be applied tothe skin of a patient 2, on the other hand by a module 3 for filteringand amplifying and electro-optically converting electric signals fromthe heart received by means of the receiving electrodes 1, disposed in ashielded housing 4 forming a Faraday cage and connected by optical meansto an apparatus 5 for visualization and/or control and, finally, by asupport body 6 of an amagnetic material, carrying the electrodes 1 andthe shielded housing 4 containing the module 3.

The electrocardiograph signals received by the electrodes 1 are, as aresult, immediately amplified and filtered with the aid of a suitablemodule 3, totally insulated from the external electromagneticenvironment and, likewise, transformed and transmitted in optical formpractically from their reception site.

According to a first characteristic of the invention, shown in FIG. 1 ofthe attached drawings, the support body 6 is comprised by a base 7 onwhich are mounted fixedly the receiving electrodes 1, protruding fromthe side 8 applied against the skin of the patient 2, and at least oneseparator or spacing element 9 maintaining rigidly the shielded housing4 containing the module 3 at a constant distance from the base 7 andhence from the skin of the patient 2.

The sole metallic element of the detector device, namely the shieldedhousing 4, will not as a result ever be in direct contact with thepatient 2, which avoids any risk of burning.

Moreover, the electrodes 1 being fixed on the base 7, the emplacement ofthese relative to each other is set by being spaced a distance which isa function of the size of the patient, and it suffices to position saidbase 7 adjacent the heart so that the electrodes 1 will be placed in asatisfactory manner.

The support body 6 can preferably be made of a material such as Teflonor polymethyl methacrylate (also known under the term "plexiglass"),having a rounded structure without ridges.

Likewise, the electrodes 1, preferably three in number, are preferablyformed of a conductive material selected from the group formed bycarbon, carbon compounds and plastic materials and insensitive toelectromagnetic fields.

According to a preferred embodiment of the invention, shown in FIG. 3 ofthe attached drawings and so as to ensure a firm positioning of theelectrodes 1 and to guarantee receipt of cardiac electric signals asclose as possible to the heart, there is provided a belt 10 or a harnessof an amagnetic material, if desired elastic, provided with quick meansfor closure and for adjustment of the length and passing through thesupport body 6 or at least an arm fixed to said support body 6.

Thus, the base 7 of the support body 6 and thus the electrodes 1 arepermanently applied forcibly against the skin of the patient in theregion of the heart.

However, any other suitable securement means could also be envisaged,provided it consists of an amagnetic material.

As shown in FIG. 2 of the attached drawings, the module 3 for filteringand amplification and conversion can be comprised, in essence, on theone hand, by high-frequency filter units 11 each associated with one ofthe electrodes 1, on the other hand by a differential amplificationcircuit 12 associated with a low-pass filter 13 and, finally, by anelectro-optical transducer 14 connected by an optical conductor 15 to anapparatus 5 for visualization and/or control, the optical transmissionof the signals being effected, for example, by frequency modulation ormodulation of the pulse widths.

Each of the high-frequency filter units 11 ensuring the interfacebetween the electrodes 1 and the module 3, could preferably be disposedin a shielded housing 16 forming a Faraday cage and the electricalconnections between said electrodes 1 and the filter units 11 of themodule 3 will be preferably comprised by rigid filaments 17 of shortlength, as the case may be shielded and each integrating a resistancelimitation 18, limiting the interferences of the externalelectromagnetic waves and avoiding any formation of loops and risk ofburning.

The differential amplification circuit 12 can have for example a gain ofabout 300 to 500 and one of the three electrodes 1 forming a part of thedetector device according to the invention, could be used for thereinjection of the common mode (the two others functioning asreceivers), permitting freedom from parasitic signals, particularly lowfrequency ones, received by the two receiving electrodes 1.

The filter 13 could preferably have a cutoff frequency of the order of20 Hz.

The apparatus 5 for visualization and/or control could be disposedeither in the enclosure of the NMR apparatus (Faraday cage), or outsidethe latter and will comprise an opto-electric reconversion unit, ascreen for visualization and/or registration and/or a module fordetection or analysis of the QRS complexes or of another parameter ofthe electrocardiogram signal, permitting triggering or controlling oneor several apparatuses for analysis or visualization and/or for testingthe patient.

The apparatus 5 could for example consist in a monitor for surveillanceof the vital physiological parameters of a patient in the course of saidNMR examination, of the type of that which is the object of Frenchapplication No. 9014846 of Nov. 23, 1990 in the name of the applicant.

According to a first modified embodiment of the invention, the module 3comprises also, for its electrical supply, a battery 19 or arechargeable long length condenser of the amagnetic type, an opticalconductor, associated with an optically controlled switch disposed inthe housing, which can permit controlling the operation and supply ofsaid module 3 for filtering and amplification and conversion and, as thecase may be, the adjustment of the different components (11 to 14) ofthis latter (not shown).

According to a second modified embodiment of the invention, the energysupply of the module 3 for filtering and amplification and conversion isprovided by means of an optical conductor coacting with a photovoltaiccell or like device disposed in the shielded housing 4.

The improvement of the quality of the electrocardiogram signalsreceived, resulting from the use of the invention, will be quiteapparent by comparing the signal curves of FIGS. 4 and 5 with eachother, these signals having been taken from patients subjected to NMRexamination by means of a whole antenna body.

Of course, the invention is not limited to the embodiment described andshown in the accompanying drawing. Modifications remain possible,particularly as to the construction of the various elements or by thesubstitution of technical equivalents, without thereby departing fromthe scope of protection of the invention.

We claim:
 1. In a detector device for electrocardiograms, adapted to beused in a loaded and sensitive electromagnetic environment, adjacent orwithin a nuclear magnetic resonance apparatus, and on a patient withinthe tunnel of the magnet of a magnetic resonance imager (MRI), theimprovement wherein the device comprises at least two non-metallicelectrodes for application to the patient's skin for receivingelectrical signals from the heart; a module including means forfiltering the electrical signals received from the heart, means foramplifying filtered signals, and means for electro-optically convertingfiltered and amplified electrical signals, said module being disposed ina shielded housing forming a Faraday cage; an optical means connectingsaid casing to an apparatus for visualization; and a support body of anamagnetic material carrying the shielded housing containing the module,said support body including means for maintaining the shielded housingat a constant distance from the patient's skin.
 2. Detector deviceaccording to claim 1, wherein the support body includes a base having aside for application against the patient's skin, the electrodes beingfixedly mounted on and projecting from said side, and said means formaintaining the shielded housing at a constant distance comprising atleast one spacing element.
 3. Detector device according to claim 1,wherein the electrodes are three in number, and are of a conductivematerial selected from the group consisting of carbon, carbon compoundsand plastic materials.
 4. Detector device according to claim 1, whereinthe shielded housing is associated with a harness of an amagneticmaterial for securement to the patient's heart region.
 5. Detectordevice according to claim 1, wherein the means for filtering comprisehigh-frequency filter units, each connected with one of the electrodes,the means for amplifying comprise a differential amplification circuitconnected to a low-pass filter, and the means for electro-opticallyconverting comprise an electro-optical transducer connected to anoptical conductor to the apparatus for visualization.
 6. Detector deviceaccording to claim 5, wherein the connections between the electrodes andthe filter units comprise filaments of short length, each filamentintegrating a limitation resistance.
 7. Detector device according toclaim 5, wherein the device comprises three electrodes.
 8. Detectordevice according to claim 1, wherein the module further comprises apower source selected from the group comprised of a battery and arechargeable long-duration amagnetic condenser, and an electricalconductor associated with an optically controlled switch disposed in thehousing for controlling the operation and the supply of said module andthe adjustment of its components.
 9. Detector device according to claim1, wherein the module is supplied with energy provided by means of anoptical conductor operatively associated with a photovoltaic celldisposed in the shielded housing.